In a machine such as a fax machine, a copier, or a hand scanner, an image reading apparatus such as an image sensor is used as an apparatus for reading an original copy. Normally, a contact-type image sensor that has a short optical path length and that can be easily incorporated into the machine is used as this kind of image reading apparatus.
A contact-type image sensor is equipped with a line illuminator that linearly illuminates an original copy surface across a main scanning area. As a line illuminator, an indirect illumination-type line illuminator is known that uses a long light guide member and that propagates a light incident from a light source disposed at an end surface of the light guide member while causing the light to be reflected by an inner surface to emit the light from a linear emitting surface in a longitudinal direction.
It is known that a white LED that includes a blue LED chip and a YAG phosphor can be used as a light source in this kind of line illuminator. A white LED light source is capable of reading color images, and lighting control thereof is simpler in comparison to a multi-chip LED that emits light of the three colors red, green and blue (RGB). Further, it is said that a white LED light source is superior in terms of brightness characteristics and with respect to miniaturization and weight reduction, and also enhances light emission efficiency when employed as the light source of a contact-type image sensor.
Further, users desire that in a line illuminator in which an LED including a plurality of light emitting elements is disposed as a light source at an end surface of a light guide member, emitted light colors from the plurality of chips are released after being sufficiently mixed in the light guide member without disturbing the color balance, and illuminate a color original copy as linear illumination with a homogeneous hue. An invention for solving these problems or that has a feature relating to the shape of a light guide member is disclosed in Patent Document 1 and the other documents.
Further, Patent Document 2 discloses that when white LEDs are mass produced, considerable variations arise with respect to the light emission wavelength and light emission brightness of blue LED chips. Furthermore, white light that is mixed is also affected by variations in the amount and dispersion of fluorescent particles of YAG phosphor that get mixed in with a covering member and the like, and it is considered that, as a result, large variations arise in the hue and brightness of completed white LEDs.
The distribution of variations in the hue of emitted lights of white LEDS that were mass produced in lots will now be described referring to FIG. 16 using one section of the chromaticity coordinates thereof. The distribution chart shown in FIG. 16 is disclosed in Patent Document 2. Each black spot illustrated in FIG. 16 represent individual hue data of a white LED, and the hue is dispersed in a band shape that slopes upward to the right as illustrated in FIG. 16. More specifically, the hue is dispersed in a band shape in a white region that falls approximately on a line that passes from blue through yellow via a white point according to the chromaticity classification of JIS Z 8110. In this case, the dispersion in the width direction (represented by the arrowed line A) is mainly due to hue variations caused by variations in the light emission wavelength of blue light emitting elements, and the dispersion in the longitudinal direction (represented by the arrowed line B) is mainly due to hue variations caused by variations in the amount or dispersion of fluorescent particles that have gotten mixed into the covering member. In this connection, since the light emission wavelength of blue light emitting elements varies significantly between lots, the dispersion in the width direction (represented by the arrowed line A) spreads further in actual mass production (see Patent Document 2).
In this case, when attempting to construct a light source of a line illuminator for an image reading apparatus using such kind of white LED, it is preferable from the viewpoint of the central hue and hue unevenness and the like that the light source is constructed using only white LEDs that have been selected by ranking the emitted light colors thereof in a desired chromaticity range. However, when a light source is constructed using only white LEDs that have a specific ranking, white LEDs with a different ranking are wasted, and there is thus the problem that the total cost for obtaining the white LEDs is comparatively high.
[Patent Document 1] Japanese Laid-Open Patent Publication No. 2006-287923
[Patent Document 2] Japanese Laid-Open Patent Publication No. 2004-119743
[Patent Document 3] Japanese Patent No. 3990437
[Patent Document 4] Japanese Laid-Open Patent Publication No. 2002-134284
[Patent Document 5] Japanese Laid-Open Patent Publication No. 2007-066969
[Patent Document 6] Japanese Laid-Open Patent Publication No. 2006-303373